The invention relates to seafloor electromagnetic detectors (receivers) for surveying for resistive and/or conductive bodies, for example for oil and other hydrocarbon reserves, or subterranean salt bodies.
FIG. 1 schematically shows a surface vessel 14 undertaking controlled source electromagnetic (CSEM) surveying of a subterranean strata configuration according to one standard technique [1]. The subterranean strata configuration in this example includes an overburden layer 8, an underburden layer 9 and a hydrocarbon reservoir 12. The surface vessel 14 floats on the surface 2 of a body of water, in this case seawater 4 of depth h meters. A submersible vehicle 19 carrying a source in the form of a horizontal electric dipole HED transmitter 22 is attached to the surface vessel 14 by an umbilical cable 16. This provides an electrical and mechanical connection between the submersible vehicle 19 and the surface vessel 14. The HED transmitter is supplied with a drive current so that it broadcasts an HED electromagnetic (EM) signal into the seawater 4. The HED transmitter is typically positioned a height of around 50 meters or so above the seafloor 6.
One or more remote receivers 25 are located on the seafloor 6. The receivers are sensitive to EM fields induced in their vicinity by the HED transmitter, and record signals indicative of these fields for later analysis.
Another type of submarine EM survey is a passive, e.g. magnetotelluric (MT), survey. These types of survey employ similar receivers to those used in CSEM surveying and shown in FIG. 1, but do not employ a controlled source to generate the EM fields (i.e. they do not employ a transmitter 22 such as shown in FIG. 1). Passive source EM surveys are instead based on detecting the response of subterranean strata to naturally occurring broadband MT waves generated in the earth's ionosphere. There are some differences in the EM fields used in an MT survey and the EM fields typically generated in a CSEM survey, most notably in terms of frequency content, but the receivers used in these types of survey are broadly similar. Indeed, in some surveys both CSEM and MT data may be collected using the same receivers.
In performing a survey such as shown in FIG. 1, the HED transmitter 22 is driven to broadcast EM signals that propagate both into the overlying water column 4 and downwards into the seafloor 6, and the underlying strata 8, 9, 12. At practical frequencies for this method, and given the typical resistivity of the respective media 4, 8, 9, 12, propagation occurs primarily by diffusion of EM fields. The rate of decay in amplitude and change in phase of the signal are controlled both by geometric spreading and by skin depth effects. Because in general the underlying strata 8, 9, 12 are more resistive than the seawater 4, skin depths in the underlying strata 8, 9, 12 are longer. As a result, electromagnetic fields measured by a receiver located at a suitable horizontal separation are dominated by those components of the transmitted EM signal which have propagated downwards through the seafloor 6, along within the underlying strata and back up to the detector, rather than directly through the seawater.
A sub-surface structure which includes a hydrocarbon reservoir, such as the one shown in FIG. 1, gives rise to a measurable change in the EM fields (amplitude and phase) measured at a receiver relative to a sub-surface structure having only water-bearing sediments. For relatively resistive bodies in water bearing sediments, such as a hydrocarbon reservoir, the EM fields are generally enhanced in amplitude and advanced in phase. This is because EM signals are less attenuated and travel faster in resistive bodies. By way of comparison, a hydrocarbon reservoir typically has a relatively high resistivity (e.g. up to 100 Ωm or even higher) compared to other subterranean strata (typically on the order of 1 Ωm or so). It is this effect on electromagnetic fields which has been used as a basis for detecting hydrocarbon reservoirs [1]. Conversely, for relatively conductive structures in water bearing sediments, such as those saturated with briney fluids, the EM fields are generally reduced in amplitude and retarded in phase. This is because EM signals are more attenuated and travel more slowly in relatively more conductive bodies.
Thus an important aspect of CSEM surveying is an ability to record EM fields at the seafloor as reliably as possible. Field measurements for marine EM surveying applications are primarily made using receivers/detectors which may be grouped into two main types. One type may be referred to as stand alone seafloor deployed detector units, and the other type may be referred to as long-wire detectors.
Long-wire detectors (also widely known as LEM instruments) have a single long (up to 3 km) antenna deployed on the seafloor behind an instrument for recording the signals picked up in the antenna. Examples of this type of detector are described by Webb [2] and Constable [3]. Although good signal to noise ratios can be achieved with these instruments, they are time consuming to deploy since they must be deep-towed through the water column and released close to the seafloor. Accordingly, stand alone seafloor deployed detector units are often preferred.
FIG. 2 schematically shows in perspective view a known stand alone seafloor deployed type detector 25. The detector is described in detail in WO 03/104844[4]. The detector is primarily described in the context of magnetotelluric (MT) surveying. However, this type of detector is also used in CSEM surveying. A similar detector is described in U.S. Pat. No. 5,770,945[5], and further broadly similar examples are described by Sinha [6] and in GB 2 402 745 [7].
The detector 25 shown in FIG. 2 may be considered to comprise four main components. The first component, the logger unit, includes a multi-channel digital data-logging processor, magnetic field post amplifier and electric field amplifiers, all contained within a first waterproof pressure case 30. The second component is a second waterproof pressure case 32 containing an acoustic navigation/release system. The third component consists of four silver-silver chloride (Ag—AgCl) electrodes 34, 35, 36, 37 mounted on the ends of four 5-meter long booms 40, 41, 42, 43, and two silver-silver chloride (Ag—AgCl) electrodes 45, 46 located at different positions along the length of vertical arm 48. The fourth unit includes four magnetic induction coil sensors (of which three are visible in FIG. 2) 51, 52, 53. All elements of the system are mounted on or attached to a corrosion-resistant frame 56 along with glass spheres 58 for flotation, and an anchor weight 60 for deployment to the seafloor.
The booms 40, 41, 42, 43 comprise 5 m lengths of semi-rigid plastic (e.g., PVC or polypropylene) pipe, with a diameter on the order of 2 inches (5.08 cm). Insulated copper wires (not shown) are run through the pipes to connect the electrodes 35, 36, 37, 38 to the amplifiers in the logger unit. Alternatively the booms 40, 41, 42, 43 may be formed from solid rods, such as fiberglass or other durable material, which have diameters on the order of 1 to 2 cm or more. In these embodiment, the electrodes 34, 35, 36, 37 are retained on the outside of their respective booms, and the insulated wires for connection to the amplifiers run along the outer surface of the rods, preferably anchored at points along the boom length using fasteners such as clamps or cable ties.
The electrodes 45, 46 on the vertical arm 48 are for detection of a vertical electric field component. The vertical arm is inserted into a mount on the frame 56 and fastened via appropriate fastening means so that it extends vertically above the frame and the electrical components of the unit. The vertical arm 48 is a substantially rigid material in the form of a pipe or rod. To obtain the desired rigidity, arm 162 is formed from polycarbonate resin or a similar durable plastic. The electrodes 45, 46 are disposed at different points along the length of the vertical arm 48 to form a vertically-oriented dipole antenna. The electrodes 45, 46 on the vertical arm are connected respectively by insulated wires and cable to the data logger included within the electronics pressure case 30.
FIG. 3 schematically shows in perspective view another stand alone seafloor deployed type detector 70. This type of detector may be seen as a variation on the detector shown in FIG. 2 and is described in detail in WO 06/026361 A1 [8]. Apart from differences in the electrode structure, the detector of FIG. 3 is otherwise the same as the detector shown in FIG. 2. The electrode structure is different in that instead of electrodes mounted on booms, the electrode structure comprises three pairs of square Ag—AgCl electrodes. The three pairs of electrodes are orthogonally arranged so that each pair is for measuring respective EM fields along two horizontal (x and y) and one vertical (z) direction. The electrodes in each pair are positioned parallel to each other such that together the six electrodes form a cuboid shape. The electrodes in each pair are connected together by a resistor (not shown in FIG. 3) having a resistance value selected to match the resistance of seawater between the electrodes. The electrodes are retained within a frame through which connectors 74 are passed to connect the electrodes to cables 72 located external to the assembly. The cables provide connections from the electrodes to their corresponding amplifiers 76 and a data-logging processor 78. This electrode configuration and the use of the resistors is said to reduce distortion of the measured electric fields in the seawater.
Conventional detectors for EM surveying suffer from a number of problems.
For short arm (boom) instruments (i.e. for stand alone seafloor deployed type detectors as shown in FIG. 2) the arms are generally only semi-rigid and so prone to flexing. The present inventors have appreciated that this makes it difficult to accurately determine the orientation of the dipole antennae comprising the detector on the seafloor. This can be problematic because valuable information regarding the subterranean strata can be obtained using full vector information on measured EM fields (i.e., by taking account of the directions of EM fields, as well as their amplitudes and phases). For example, directional information is important both for characterizing subterranean strata having variations within horizontal layers, and also for distinguishing different background strata configurations, even in cases where the strata are largely one-dimensional (horizontally layered). This is because directional information allows transverse electric (TE) and transverse magnetic (TM) modes in the transmitted fields (which modes are differently sensitive to different subterranean strata configurations) to be distinguished at the detector [1]. Furthermore, inaccurate orientation information causes problems in accurately determining spatial gradients in EM fields which are important in some analysis schemes because they are particularly sensitive to lateral structural variations in subterranean strata, and may also be used to de-convolve measured fields into “pure” TE and “pure” TM components which are more amenable to some types of further analysis, e.g., as described in GB 2 411 006 A [9] and GB 2 423 370 [10] (the raw EM field data are in general mixed mode for arbitrary source orientations and detector locations).
Accurate orientation information is difficult to obtain for detectors of the type shown in FIG. 2 because while the orientation of the main body of the unit may be determined using an appropriate on-board compass device, the booms themselves will typically have flexed during their descent through the water column. Because of this when the detector units and their booms come to rest on the seafloor, the electrodes at their ends are moved away from their assumed (nominal) positions with respect to the frame of the receiver. This means the orientation of the receiver dipoles provided by the electrodes is typically not known to an accuracy of any better than 5 degrees or so, whereas 1 degree accuracy or better is desired for accurate use of orientation information.
Furthermore, the semi-rigid nature of the booms of detectors such as shown in FIG. 2 renders them prone to motionally induced noise. Boom motion induces noise in two ways. Firstly, it causes changes in the direction along which electric fields are measured, and secondly the movement of the antennae and their associated cabling through the Earth's magnetic field induces electric fields in the measurement channels. These effects can mean motionally induced noise becomes the dominant source of noise, especially in shallow water and areas with extreme seafloor currents.
A further problem with known marine EM surveying detectors arises from the fact that the signals to be measured are extremely small. For example, electric fields at the detector are typically on the order of only a nanoVolt/meter or so. This can be particularly problematic for stand alone seafloor deployed type detectors such as shown in FIGS. 2 and 3 since the relatively small scale of these detectors (compared to long-wire detectors) means the voltage difference between their electrodes will typically be only 10 nV or so (for electrodes separated by 10 m). The difficulty in measuring such small voltages is exacerbated by the remote location and the hostile environment in which detectors are located. Thus the signals to be measured can easily become contaminated by noise arising from connectors and cabling to the extent that the signals can become completely lost in noise if the connectors become worn or corroded by seawater.
What is more, the field components measured at the detector along different direction are likely to differ significantly from one another, with signals on the order of a nanoVolt/meter only likely for the strongest signal components. Other signal components can be much weaker. For example, the detectors shown in FIGS. 2 and 3 measure signals in two orthogonal horizontal directions and a vertical direction. For the electric fields in a marine EM survey, the fields at a detector will predominantly be in a generally horizontal direction. The particular direction within the horizontal plane will depend primarily on the nature and direction of the source of the fields relative to the detector. The relative intensities of the three field components measured at a detector will depend on the orientation of the detector with respect to the direction of the induced fields in its vicinity. For example, the vertical component will almost always be particularly weak because there is generally little vertical signal. The vertical component might, for example, by a number of orders of magnitude weaker that the horizontal components. Furthermore, one or other of the measured horizontal components may be similarly weak if the detector is oriented with one of its antenna aligned closely with the direction of the EM fields (leading to a relatively strong signal component), and one of its antennae orthogonal thereto (leading to a relatively weak signal component). This variation in the signal strengths associated with different spatial components means there is a corresponding wide range in associated signal-to-noise ratios. When combining signals from the three spatial directions to obtain a resultant measure of the fields at the detector, the overall accuracy can be strongly affected by the poor signal-to-noise ratio in the weakest signal component, thus reducing the overall accuracy of the measurement.
Accordingly there is a need for a detector for marine EM surveying which is easier to deploy than known long-wire type detectors, but which does nor suffer the above-mentioned drawbacks of known stand alone seafloor deployed type detectors.